Hmm, you want to know everything about the design of toe drains. They teach whole graduate course in geotechnical engineering on the subject of subsurface drainage. You need to have an estimate of how much water per unit length that must be collected during the peak wet conditions, the native soil parameters (particle size distribution, etc.), lengths to convey collected water to point of outfall, some information about the structure being drained (like shape and features that effect drainage paths), etc..